Improved hierarchical term handling

[dcavins]

This PR adds handling for hierarchical terms: • Make sure that newly added terms are added to the $exists array and found by WXR_Importer::term_exists(). • Basic term parent setting when the parent exists before the child term is created. • Deferred term parent setting when the child term is encountered before the parent term exists.

Please let me know if you have any questions.

This is a great project, thanks for taking it on!